This invention relates to the reactivation of noble metal or noble metal oxide coated anodes that have been deactivated. In particular, it relates to coating deactivated anodes with a noble metal such as platinum to reactivate them.
Anodes are used in the electrolytic production of caustic soda, chlorine, sodium chlorate, and other products. A typical industrial anode consists of a titanium substrate that is coated with a mixture of noble metals or a mixture of a noble metal oxide and a valve metal oxide. After a period of use, the anodes become deactivated or passivated, and more and more voltage is required to obtain the same output of product. When the anode potential exceeds 1.4 volts versus SCE (saturated calomel electrode) in a saturated brine solution, the anode is considered to be deactivated to the extent that it is more economical to replace or recoat it than to continue using it.
At the present time, deactivated anodes are refurbished by sandblasting the noble metal or metal oxide coating off the substrate, etching the substrate with hydrochloric acid or oxalic acid to remove surface oxides of titanium, and applying a fresh coating to the substrate. To apply a coating of, for example, RuO.sub.2 /TiO.sub.2, the substrate is painted with a mixture of ruthenium trichloride and butyltitanate in water or isopropanol until a coating forms. The coating is then heated to form the oxides. This procedure is repeated as many times as is necessary to obtain a coating about 10 to about 15 microns thick or a noble metal loading of about 3 to about 15 g/m.sup.2. Considerable expense is involved in producing a reactivated anode by this procedure.